The present invention relates to an optical element such as a lens, a prism, and a reflecting mirror provided with a multilayered optical thin film on its surface. The present invention also relates to an exposure apparatus provided with the optical element as described above.
A variety of optical thin films such as reflection films and anti-reflection films are applied to the optical element for constructing an optical system such as a lens, a prism, and a reflecting mirror. For example, the anti-reflection film is applied in order to reduce undesirable reflection. On the other hand, the reflection film is applied to the surface of the optical element in order to efficiently reflect the incident light on the surface of the reflection film.
Such an optical thin film is generally produced in accordance with the dry process. The dry process includes the vacuum deposition, the sputtering, CVD (Chemical Vapor Deposition). The dry process is described, for example, in Joy George, Preparation of Thin Films (Marcel Dekker, Inc., New York, 1992) and Francois R. Flory, Thin Films for Optical Systems (Marcel Dekker, Inc., New York, 1995).
The anti-reflection film is required to have such performance that the reflectance is low over a wide range of angle of incidence. The reflection film is required to have such performance that the reflectance is high with satisfactory angle-dependent characteristics over a wide range of wavelength. In order to respond to the request for the performance as described above, it is known that a multilayered film is formed in a well-suited manner by combining a plurality of coating materials having different refractive indexes. Further, as for the multilayered film, it is known that the larger the difference in refractive index among a variety of coating materials to be used is, and the lower the minimum refractive index of those of the variety of coating materials is, the more the optical performance of the multilayered film is improved. Further, it is also known that the number of coating layers can be decreased by using coating materials which are greatly different in refractive index in combination, and using a coating material which has an extremely low refractive index. As a result, an optical thin film, which has high performance in relation to the light beam in the visible region, is obtained.
The integration is highly advanced and the function is highly progressive for ULSI in the exposure apparatus for semiconductors. An optical system such as a projection lens thereof is required to have a high resolution and a deep depth of focus in order to successfully obtain a machining line width of 0.18 xcexcm. The projection lens is used to project a device pattern on a photomask onto a wafer so that the wafer is exposed therewith. The resolution and the depth of focus of the projection lens are determined by the wavelength of the light used for the exposure and N.A. (numerical aperture) of the projection lens.
In general, as for the device pattern on the photomask, the higher the definition is, the larger the angle of diffraction of the diffracted light is. Therefore, in order to perform the exposure with such a pattern, the diffracted light may be fetched by using a projection lens having large N.A. Further, the angle of diffraction of the diffracted light from the pattern is decreased when the light has a shorter wavelength xcex. Therefore, it is also advantageous to use the light beam having a short wavelength for the exposure of the pattern having such a definition as described above.
The resolution and the depth of focus are represented by the following expressions respectively.
Resolution=k1(xcex/N.A.)xe2x80x83xe2x80x83(1)
Depth of focus=k2{xcex/(N.A.)2}xe2x80x83xe2x80x83(2)
(In the expressions, k1 and k2 are proportional constants.)
Therefore, in order to improve the resolution (decrease the value), N.A. may be increased, or xcex may be shortened. However, if N.A. is increased, the depth of focus is shortened, as appreciated from the expression of the depth of focus. When the depth of focus of the optical element such as the projection lens is shortened in the semiconductor exposure apparatus, the throughput is affected thereby. Therefore, in order to improve the resolution, it is more preferred to shorten xcex rather than N.A. is increased. From such a viewpoint, the wavelength of the exposure light beam is progressively shortened, from the g-ray (436 nm) to the i-ray (365 nm) and further to the excimer laser beams such as KrF (248 nm) and ArF (193 nm).
In spite of the trend to realize the exposure with the short wavelength as described above, it has been hitherto extremely difficult to obtain a high performance optical thin film for an ultraviolet light source, for example, those used in the vicinity of 200 nm, unlike those obtained in the visible region, because of the following reason. That is, many coating materials absorb the light in this wavelength region, resulting in light loss. The coating material, which can be used in the ultraviolet region in the vicinity of 200 nm as described above, is extremely restricted. Therefore, it is difficult to sufficiently increase the difference in refractive index between the coating materials as described above, and it is difficult to extremely decrease the minimum refractive index of those of various coating materials. Therefore, it has been hitherto extremely difficult to design and produce a high performance optical thin film to be used in such a wavelength region.
At present, it is possible to use a variety of anti-reflection film materials in order to produce a typical anti-reflection film to be used for the light in the visible region by means of the dry process. In general, in the visible region, TiO2 (n=2.4 to 2.7 at 500 nm) is used as the maximum refractive index material, and MgF2 (n=1.38 at 500 nm) is used as the minimum refractive index material (n represents the refractive index). However, only a few coating materials are usable for the ultraviolet light having the wavelength in the vicinity of 200 nm. In general, the refractive index n is about 1.7 (n=about 1.7) in relation to the wavelength of 200 nm for any one of LaF2, NdF3, and GdF3. These materials are usable coating materials having the maximum refractive index. The refractive index n is 1.36 (n=1.36) in relation to the wavelength of 200 nm for Na3AlF6. This material is a usable coating material having the minimum refractive index. Therefore, the difference in refractive index among a plurality of coating materials used for the light at the wavelength of 200 nm is by far smaller than the difference in refractive index among a plurality of coating materials used for the light in the visible region.
The coating material, which is usable in the ultraviolet region, is extremely limited as described above. Therefore, those skilled in the art will understand the fact that the design and the production of the optical thin film are more difficult in the ultraviolet region than in the visible region.
It is known that the optical thin film is produced in accordance with the wet process. For example, a thin film can be produced by means of hydrolysis and polymerization with a metal alkoxide solution, i.e., a liquid. The wet process is called xe2x80x9csol-gel processxe2x80x9d. As well-known in the art, for example, SiO2, ZrO2, HfO2, TiO2, and Al2O3 can be produced not only by the dry process but also by the sol-gel process. The method is disclosed, for example, in Ian M. Thomas, Applied Optics Vol. 26, No. 21 (1987) pp. 4688-4691 and Ian M. Thomas, SPIE Vol. 2288 Sol Gel Optics III (1994) pp. 50-55. In the case of the SiO2 film formed by the sol-gel process, a colloidal SiO2 suspension, which is appropriate to manufacture the SiO2 film, is usually prepared by means of hydrolysis of silicon alkoxide in base alcohol as a solvent. Hydrolysis of tetraethyl silicate in ethanol can be represented, for example, by the following formula (3).
Si(OC2H5)4+2H2Oxe2x86x92SiO2+4C2H5OHxe2x80x83xe2x80x83(3)
This reaction is complicated, in which characteristics of the product are affected by various parameters including, for example, the catalyst, the water ratio, and the temperature. Usually, three types of the liquid coating methods, i.e., the spin coat method, the dipping method, and the meniscus method are used to execute the wet process coating. The dipping method is suitable for a large-sized substrate having an irregular configuration or a curved surface. The spin coat method is suitable for a small-sized substrate having a round configuration, a flat surface, or a gentle curvature. The meniscus method is especially suitable for a large-sized flat surface substrate. These techniques are disclosed, for example, in xe2x80x9cSol-Gel Science; Academic Press, Inc., Sandiego, 1990xe2x80x9d written by Brinker and Sherer and xe2x80x9cThin Solid Films, Vol. 175 (1989) pp. 173-178xe2x80x9d by Floch, Priotton et al.
When the wet process as described above is used, it is possible to obtain any one of a film having a high filling density and a film having a low filling density. In order to obtain a film having a high filling density equivalent to the film formed by the dry process, by means of the wet process, it is generally required to heat the film at a high temperature (for example, not less than 450xc2x0 C.) in the production step. This requirement involves such a fear that not only the extension of the production time and the increase in production cost but also the damage and the deterioration of the substrate may be caused. Therefore, the wet process is disadvantageous as compared with the dry process. On the other hand, the wet process such as the sol-gel process is performed at the room temperature or at a temperature of not more than 150xc2x0 C. Therefore, it is unnecessary to perform any additional step such as the heating step at a high temperature. Accordingly, it is easy to successfully obtain a film having a low filling density.
The present inventors have disclosed, in Japanese Laid-Open Patent Publication No. 10-319209 (U.S. Pat. No. 5,993,898 corresponding thereto), a method for producing an anti-reflection film and a reflection film in which an optical thin film formed by the wet process and a thin film formed by the wet process are used in combination. In this method, the film having a low refractive index, which is not obtained as a film formed by the ordinary dry process, can be formed in accordance with the wet process. Further, the film, which has a high refractive index, can be formed in accordance with the dry process. Therefore, it is possible to form a multilayered thin film which has a large difference in refractive index among multilayered films and which has a low refractive index layer with an extremely low refractive index.
In general, the thin film can be recognized as a model of a structural body in which a plurality of minute pores are separated from each other by a solid substance. Therefore, the relationship between the filling density and the refractive index of the film is represented as follows.
nf=n0xc3x97p+npxc3x97(1xe2x88x92p)xe2x80x83xe2x80x83(4)
In this expression, np represents the refractive index of the substance (for example, air or water) with which the minute pores are filled, nf and n0 represent the actual refractive index (depending on the filling density) and the refractive index of the deposited solid material respectively, and p represents the filling rate of the film. Further, the filling rate is defined as follows.
p=(volume of solid portion of film)/(total volume of film)xe2x80x83xe2x80x83(5)
In this expression, the total volume of the film corresponds to the total sum of the volume of the solid portion of film and the volume of the minute pore portion of the film.
Accordingly, the high filling density and the low filling density mean the high refractive index and the low refractive index respectively. In the case of the SiO2 film produced by the wet process, the filling rate can be varied from 1 to about 0.5. Therefore, the refractive index can be changed in the visible region from 1.45 to 1.22. As a result, a monolayer anti-reflection layer, which has a reflectance of almost 0%, can be formed on the optical glass by using the wet process SiO2 having the low filling density. As for the monolayer anti-reflection layer, the reflectance can be approximately 0% in the case of the perpendicular incidence. However, the monolayer anti-reflection layer involves such a problem that the reflectance is increased in the case of the oblique incidence.
It is generally known that ammonia is added as a catalyst to the hydrolysis reaction represented by the formula (3) described above, as a method for producing an SiO2 film having a low filling density and a high purity in which the refractive index is 1.22 that is low in the visible region. Owing to the catalytic action of ammonia, it is possible to prepare a suspension including minute spherical SiO2 particles having a high purity. This suspension is applied onto the substrate surface, and the alcohol solvent is evaporated at the room temperature. Accordingly, it is possible to prepare a porous SiO2 film composed of spherical SiO2 particles, i.e., an SiO2 film having a low filling density. As well-known in the art, the anti-reflection film, which is composed of the SiO2 film having the low filling density, has high durability against the laser. Therefore, the anti-reflection film is used, for example, for the high output laser such as those used for the nuclear fusion. This technique is described in Ian M. Thomas, Applied Optics Vol. 31, No. 28 (1992) pp. 6145-6149.
An object of the present invention is to solve the problems involved in the conventional technique as described above and provide an optical element such as a lens, a prism, and a reflecting mirror which can be used in the ultraviolet region of not more than 300 nm and which is provided with a high performance multilayered optical thin film, especially, a multilayered anti-reflection film or a multilayered reflection film. Another object of the present invention is to provide a projection exposure apparatus which is provided with the optical element as described above.
Still another object of the present invention is to provide an optical element provided with a multilayered anti-reflection film in which the reflectance is low over a wide range of angle of incidence and the difference in reflection characteristic is small irrelevant to the direction of polarization. Still another object of the present invention is to provide an optical element provided with a multilayered reflection film which has a reflectance of not less than 97% for any one of the p-polarization and the s-polarization over a wide wavelength region in the case of oblique incidence.
Still another object of the present invention is to provide a multilayered thin film-equipped optical element which is used together with a light beam in an ultraviolet region of not more than 300 nm, especially in a wavelength region of not more than 250 nm in which N.A.xe2x89xa70.80 (not less than 0.8) is satisfied, and a high resolution exposure apparatus provided with the optical element as described above.
According to a first aspect of the present invention, there is provided an optical element comprising:
an optical substrate; and
a multilayered optical thin film which is formed on the optical substrate, wherein:
a refractive index of at least one layer of the multilayered optical thin film with respect to a light beam having a wavelength of not more than 250 nm is not more than 1.35.
In the optical element of the present invention, the refractive index of at least one layer for constructing the multilayered optical thin film with respect to the light beam having the wavelength of not more than 250 nm is an extremely low refractive index, i.e., not more than 1.35. Accordingly, it is possible to increase the difference in refractive index among a plurality of thin films. Therefore, even when the optical element is used with the light beam having the wavelength of not more than 250 nm, i.e., the light beam having the short wavelength including, for example, those at 248 nm (KrF), 193 nm (ArF), and 157 nm (F2) as the oscillation wavelength of the excimer laser, the optical element exhibits satisfactory values for optical characteristics such as the reflectance (anti-reflection), the polarization characteristic, and the dependency on the angle of incidence.
The refractive index of the at least one layer of the multilayered optical thin film with respect to the light beam having the wavelength of not more than 250 nm is preferably 1.10 to 1.35, and especially preferably 1.15 to 1.25.
The at least one layer of the optical element of the present invention may be formed by using the wet process. Especially, when the at least one layer is formed by means of the sol-gel method, a thin film having a low filling rate, i.e., a low refractive index is obtained. Therefore, this procedure is advantageous. The at least one layer is preferably composed of fluoride of alkaline earth metal or an oxide of silicon. Especially, an MgF2 layer is preferred.
The multilayered optical thin film, which is formed on the optical element of the present invention, may be an anti-reflection film or a reflection film. When the multilayered optical thin film is used as the anti-reflection film, it is desirable that the anti-reflection film has a reflectance of not more than 0.5 with respect to light beams having short wavelengths of not more than 250 nm, including, for example, wavelengths of 157 nm, 193 nm, and 248 nm, provided that the angle of incidence is not more than 55 degrees. The reflectance referred to herein means an average value of reflectances for the s-polarized light and the p-polarized light. The optical element such as a lens of N.A.xe2x89xa70.80 has a high curvature. Therefore, it is advantageous to form such an anti-reflection film on the surface of the optical element, because a low reflectance is exhibited over a wide range of angle of incidence. Further, when the multilayered thin film is used as an anti-reflection film, the reflectance is preferably not more than 0.3%, especially preferably not more than 0.2% with respect to the light beam having the wavelength of not more than 250 nm including, for example, wavelengths of 157 nm, 193 nm, and 248 nm, provided that the angle of incidence is not more than 55 degrees.
When the multilayered optical thin film is used as a reflection film, it is desirable that the reflectance is not less than 97% with respect to the light beam having a wavelength of 193 nm.
It is preferable that the optical element of the present invention is used together with an ultraviolet light beam having a wavelength of not more than 300 nm, preferably not more than 250 nm, and more preferably not more than 200 nm. In this case, it is preferable that the optical substrate of the optical element is formed of fluorite or quartz glass.
Typically, the optical element is, for example, a lens, a prism, or a reflecting mirror. Especially, the optical element is preferably used for a projection lens to be used for a projection exposure apparatus for performing exposure with a minute pattern based on the use of an ultraviolet light beam as described above, especially for a projection lens which satisfies N.A. (numerical aperture)xe2x89xa70.80.
According to a second aspect of the present invention, there is provided an exposure apparatus for exposing a substrate with an image of a pattern on a mask, the exposure apparatus comprising:
an illumination optical system which illuminates the mask with a vacuum ultraviolet light beam;
a projection optical system which includes an optical element and which projects the image of the pattern on the mask onto the substrate; and
a multilayered optical thin film which is formed on a surface of the optical element, wherein:
a refractive index of at least one layer of the multilayered optical thin film with respect to a light beam having a wavelength of not more than 250 nm is not more than 1.35.
According to a third aspect of the present invention, there is provided an exposure apparatus for exposing a substrate with an image of a pattern on a mask, the exposure apparatus comprising:
an illumination optical system which includes an optical element and which illuminates the mask with a vacuum ultraviolet light beam;
a projection optical system which projects the image of the pattern on the mask onto the substrate; and
a multilayered optical thin film which is formed on a surface of the optical element, wherein:
a refractive index of at least one layer of the multilayered optical thin film with respect to a light beam having a wavelength of not more than 250 nm is not more than 1.35.
Each of the exposure apparatuses according to the second and third aspects of the present invention is provided with the optical element in which the multilayered optical thin film including the layer having the refractive index of not more than 1.35 with respect to the light beam having the wavelength of not more than 250 nm is formed on the surface. Therefore, the optical characteristics such as the reflection and the anti-reflection of the optical element are satisfactory when the vacuum ultraviolet light beam, especially the light beam having the wavelength of not more than 250 nm is used as the light beam for the exposure. As a result, it is possible to highly accurately expose the substrate with the fine mask pattern.
It is preferable for the exposure apparatus that the multilayered optical thin film is an anti-reflection film, and the anti-reflection film has a reflectance which is not more than 0.5% with respect to the light beam having a wavelength selected from the group consisting of wavelengths of 157 nm, 193 nm, and 248 nm, provided that an angle of incidence is not more than 50 degrees.
It is desirable for the exposure apparatus according to the third aspect that the projection optical system includes at least one projection lens, a multilayered optical thin film is formed on a surface of the projection lens, and a refractive index of at least one layer of the multilayered optical thin film with respect to a light beam having a wavelength of not more than 250 nm is not more than 1.35.
In the exposure apparatus according to the present invention, the optical element of the projection optical system may be a projection lens or a reflecting plate. When the projection optical system is provided with the reflecting plate such as a mirror, the multilayered thin film may function as a reflection film. When the projection optical system is provided with the projection lens, the multilayered thin film may function as an anti-reflection film. In the case of the latter, the projection optical system usually includes a plurality of projection lenses. It is advantageous that the multilayered thin film according to the present invention is applied to the lens disposed at the position closest to the light-outgoing side (wafer side). The exposure apparatus, to which the present invention is applicable, includes arbitrary projection exposure apparatuses such as the full field exposure apparatus, the scanning type projection exposure apparatus, and the mirror projection type exposure apparatus.